GEOPHYSICAL RESEARCH LETTERS, VOL. 40, 1513–1516, doi:10.1002/grl.50295, 2013 Variations in Mid-Continent Rift magma volumes consistent with microplate evolution Miguel Merino,1 G. Randy Keller,2 Seth Stein,1 and Carol Stein3 Received 19 December 2012; revised 21 February 2013; accepted 23 February 2013; published 28 April 2013. [1] Modeling of gravity data along the ~1.1 Ga failed Mid- formation of the MCRS. Volcanic activity was followed by Continent Rift System shows systematic patterns in magma deposition of clastic sediments in subsiding basins and volume between and along the rift’s two arms. The volume subsequent faulting of these lithified sediments [Halls, of magma increases towards the Lake Superior region, 1982; Wold and Hinze, 1982]. Eventually, changing far-field consistent with magma flowing away from a hotspot stresses, as the Grenville orogeny progressed, are thought to source there. The west arm experienced significantly more have caused compression that slowed and stopped the magmatism. These patterns are consistent with a model in extension, leaving a failed rift [Cannon, 1994]. which the two rift arms acted as boundaries of a [3] The 2000 km long MCRS, which is comparable in microplate. The volume of magma along the west arm length to the presently active East African and Baikal rifts, increases with distance from the Euler pole, indicating that has two major arms meeting in the Lake Superior region. it acted essentially as a spreading ridge, whereas the much One extends southwestward at least as far as Kansas, and smaller magma volumes along the east arm are consistent the other extends southeastward at least through Michigan. with its acting as a leaky transform. This view of the rift Because the rift is hidden beneath Phanerozoic sedimentary system’s evolution is compatible with the rift being part of rocks except where it is exposed in the Lake Superior region, an evolving plate boundary system rather than an isolated its location and geological characteristics are primarily episode of midplate volcanism. Citation: Merino, M., G. R. inferred from the gravity and magnetic anomalies, extrapola- Keller, S. Stein, and C. Stein (2013), Variations in Mid-Continent tions from the outcrop area, seismic reflection profiles, and a Rift magma volumes consistent with microplate evolution, few basement drill holes. Geophys. Res. Lett., 40, 1513–1516, doi:10.1002/grl.50295. [4] Early seismic studies indicate that the crust beneath Lake Superior and portions of the west rift arm is thickened and anomalously dense [Ocola and Meyer, 1973]. Similar 1. Introduction crustal thickening was found in the east arm by Halls fl [2] The Mid-Continent Rift System (MCRS) is one of the [1982]. Seismic re ection data from the GLIMPCE program most prominent features on the Bouguer gravity map of the of active source studies across Lake Superior [Cannon et al., central United States (Figure 1). The rift formed at ~1.1 Ga, 1989; Shay and Trehu, 1993] show that the crust was ini- recorded by two pulses of magmatic activity lasting ~15 Myr tially thinned to about one fourth of its original thickness. [White, 1997], making it one of the most extensive paleorifts The resulting basin was filled with extrusive volcanics and in the world [Hinze et al., 1997]. Petrologic and geochemical sediments, and volcanic underplating, producing a rift models favor the MCRS having been formed in the conti- pillow, subsequently thickened the lower crust. Such crustal nental interior by a mantle plume [Davis and Green, 1997; rethickening has been identified in other rifts [Thybo and Nicholson et al., 1997; Vervoort et al., 2007]. Alternatively, Nielsen, 2009]. Soon after magma had stopped erupting, many tectonic models view the rift as having formed as a the normal faults were inverted to reverse motion, presum- part of the Grenville orogeny [McWilliams and Dunlop, ably due to the Grenville orogeny [Cannon, 1994]. 1978; Gordon and Hempton, 1986], which is the series of [5] The highly magnetic and dense mafic igneous rocks 1.3–0.9 Ga tectonic events associated with the assembly of filling the rift basin were juxtaposed by high-angle reverse Rodinia [Whitmeyer and Karlstrom, 2007]. In such interpre- faulting against the less magnetic and less dense clastic tations, northwest-directed convergence at the southern rocks deposited in the basins that originally overlaid them margin of Laurentia (Proterozoic North America) caused [King and Zietz, 1971]. The resulting gravity and magnetic extension and magmatism to the northwest, including anomalies have been used to map the west arm of the rift, which extends into southern Kansas and perhaps to southern All Supporting Information may be found in the online version of this Oklahoma [Adams and Keller, 1996]. Gravity and magnetic article. 1 anomalies also show that the rift continues into the basement Department of Earth and Planetary Sciences, Northwestern University, beneath the Michigan basin [Oray et al., 1973]. This inter- Evanston, Illinois, USA. fi 2ConocoPhillips School of Geology and Geophysics, University of pretation has been con rmed by drilling in the Michigan Oklahoma, Norman, Oklahoma, USA. basin that encountered a thick section of clastic sedimentary 3Department of Earth and Environmental Sciences, University of Illinois rocks underlain by mafic volcanic rocks [Sleep and Sloss, at Chicago, Chicago, Illinois, USA. 1978] and reflection seismic studies [Brown et al., 1982] that Corresponding author: M. Merino, Department of Earth and Planetary detected the graben structure sampled by the deep drill hole. Sciences, Northwestern University, Evanston, IL 60208, USA. The southern limit of the east arm is generally placed in ([email protected]) southeast Michigan, but a series of N-S trending gravity ©2013. American Geophysical Union. All Rights Reserved. maxima that extend into Ohio, Kentucky, and Tennessee 0094-8276/13/10.1002/grl.50295 may be continuations of this arm [Halls, 1978; Keller 1513 MERINO ET AL.: VARIATIONS IN MID-CONTINENT RIFT MAGMA into the rift system’s evolution. Numerous 2-D gravity and magnetic models along parts of the MCRS have been devel- oped [Hinze et al., 1982; Wold and Hinze, 1982; Van Schmus and Hinze, 1985; Cannon et al., 1989; Woelk and Hinze, 1991; Hinze et al., 1992; Thomas and Teskey, 5 1994]. However, these models were constructed using a East Arm variety of software and modeling schemes, making it 4 6 difficult to compare results from different profiles. Hence, 7 we conducted consistent modeling across both arms of the 9 rift, allowing direct comparisons. West Arm 8 [7] The gravity data (Figure 1) were compiled from the 3 PACES database for land areas [Keller et al., 2002, 2006; 2 Hinze et al., 2005] and TOPEX satellite data for the Great Lakes [Sandwell and Smith, 2009]. Only the Bouguer 1 anomaly land data were used to create gravity models. [8] Gravity profile locations were selected to give good spatial coverage of the rift arms and, when possible, correlate with previous seismic reflection and gravity profiles. However, the seismic data have poor resolution in the lower crust; hence, they do not significantly impact Figure 1. Bouguer anomaly gravity map of the central our gravity models. Although the Lake Superior region of United States. White lines represent gravity profile and the MCRS has a significant amount of seismic data, it model locations, which are numbered and cross the anoma- was not modeled because the gravity data do not show lies that delineate the rift system. Lines 1, 2, 5, 8, and 9 a simple trend along the rift. This choice also avoided are located near seismic lines. the need to merge the higher quality land data with TOPEX satellite data. et al., 1982]. Lidiak and Zietz [1976] also suggested the [9] We used a generalized model inspired by a COCORP presence of related rifts in the eastern Kentucky area. seismic reflection line in Kansas [Serpa et al., 1984], as reinterpreted by Woelk and Hinze [1991]. This model has mafic intrusions, a sedimentary basin overlying a large 2. Gravity Analysis basaltic body, and large flanking sedimentary basins. [6] We examine variations in the volume of magmatic Thomas and Teskey [1994] infer that sediment densities in rocks along the east and west arms to seek additional insight the northern MCRS range from 2.25 to 2.66 g/cm3 depending Line 3 Line 6 100 30 50 20 0 10 mGal 0 mGal -50 -10 0 0 10 10 20 20 30 30 Depth (km) Depth (km) 40 40 50 50 0 50 100 150 0 30 60 90 Distance (km) Distance (km) Line 4 Line 8 60 30 20 0 0 mGal mGal -30 -20 0 0 10 10 20 20 30 30 Depth (km) Depth (km) 40 40 50 50 0 40 80 120 0 50 100 150 Distance (km) Distance (km) Basement, Density = 2.67 Sediments, Density = 2.55 Michigan basin sediments Density = 2.63 Mafics, Density = 3.00 Sediments, Density = 2.63 Figure 2. Four representative gravity models of the nine computed, two each from the east and west arms. The other profiles are shown in supplemental Figure 1. Black dots are observed gravity; black line is calculated gravity. 1514 MERINO ET AL.: VARIATIONS IN MID-CONTINENT RIFT MAGMA Mid-Continent Rift Magma Variation Mid-Continent Rift Magma Variation 1600 1600 A West arm B West arm ) ) East arm 2 1400 East arm 2 1400 3 3 1200 1200 5 5 1000 1000 4 4 2 2 800 800 7 7 6 6 600 8 600 8 9 1 1 9 400 400 Cross Sectional Area (km Cross Sectional Area (km 200 200 0 0 0 200 400 600 800 1000 1200 0 500 1000 1500 2000 2500 Distance From Presumed Lake Superior Hotspot (km) Distance From Euler Pole (km) Figure 3.